Wrenches (which are also often known as spanners in some countries) will only do a job properly if it’s the right size for the nut or the bolt to be turned.

The size used to describe a wrench is the distance across the flats of the nut or bolt to be turned. There are two systems in common use, metric, in millimeters, and imperial, in inches. Each of the systems has a range of wrenches especially made for each one. The systems can be identified on the wrench by either a number for metric spanners, or a fraction followed by A/F.

Another system once widely used in the United Kingdom was the Whitworth system. It used fractions but they did not refer to the distance across the flats of the fastener. Some older British and Australian machines use Whitworth-size fasteners. Some Whitworth sizes are not interchangeable with metric or Imperial systems.

There are many types of wrenches.

Choosing the correct one to use to do the job usually depends on two things: How tight is the fastener? In other words how much force is going to be applied to it, and, how accessible is it? - how much room is there to get the wrench onto the fastener, and then turn it.

When being used it's always possible a wrench will slip. Always try to anticipate what will happen if it does before putting a lot tension onto it. If possible, always pull a wrench towards you rather than pushing it away.

Ring Wrenches grip a fastener at the corners just like a socket wrench, just the sort of grip that is needed if a nut or bolt is very tight. Ring wrenches have different sized heads at each end. They aren't as convenient as sockets but can fit into places that a socket can't. One disadvantage of the ring wrench is that it can be awkward to use once the nut or bolt’s been loosened.

Open end wrenches slip easily and quickly onto fasteners, and that's particularly important for nuts and bolts in awkward places. The angle on the head allows it to be used in two different positions. While an open-end wrench often gives the best access to a fastener, if it's extremely tight the open-end shouldn't be used as this type of wrench only grips across two flats. If the jaws flex slightly or the flats don't fit tightly between them, the spanner can suddenly slip when force is applied.

The best way is to use a ring wrench to break such a bolt or nut free, then the open-end. The open-end wrench should only be used on fasteners that are no more than firmly tightened.

The combination wrench provides the user with the best features of each of the other types. It has a ring on one end for gripping and breaking the fastener's hold, and an open ended wrench of the same size on the other end.

A variation on the open end head is the flare nut wrench. It gives a better grip because the flats meet on 5 sides, not 2. The open 6th side lets the wrench be used on nuts and fittings associated with pipes and tubing. Don't use the flare-nut wrench on extremely tight fasteners as the jaws may spread, damaging the nut.

There is another type of universal adjustable open end wrench in use. Usually referred to as an adjustable wrench or simply "adjustables", the lower jaw can be moved to fit any fastener size within the wrench range. Adjustable wrenches should only be used if the correct sized wrench is not available. Both the fastener and wrench could be damaged if they are used on really tight bolts or nuts.